Many devices are known for aligning teeth, and include devices that are permanently fixed with respect to the teeth until treatment is completed, and removable devices that are designed to be worn part time or most of the time, day or night. The former are typically in the form of brackets that are bonded to individual teeth using a suitable adhesive, and a wire urges the teeth towards a final position to effect alignment. The latter are in the form of devices which fit in the intraoral cavity in a manner such as to urge teeth in a desired direction, and which are easily removable and refittable by the patient. The present invention is concerned with such removable devices.
In a first stage for providing such removable devices, a physical plaster model of the patient's dentition is made. The procedure typically requires taking an impression of the teeth to form a negative mold of the teeth, into which a plaster material is poured and set to provide a positive model of the teeth. The positive model is typically dimensionally quite accurate, and faithfully duplicates the anatomy of the patient's intraoral cavity.
Using such a model, which is typically referred to as a study model, the orthodontist is able to study the features of the patient's dentition and to devise a treatment plan to correct any malocclusion or other misalignment. For the purpose of devising such a plan, further models may be produced from the original negative mold, and these models, referred to as working models, may be used for customizing certain orthodontic appliances specifically for the patient. For example, such a working model may be used for bending and positioning wires with respect to teeth, and then for welding the same in place or with respect to a stationary device.
Typically, such appliances comprise an active element that actively generates corrective forces, and a passive element that is designed to remain stationary, serving as an anchor for the active element. Such active elements may comprise, for example, springs that generate orthodontic forces to the teeth or orthopedic forces to the malleable bony structures, and are positioned within a working model during fabrication of the appliance. Ideally, the springs are positioned such that each spring will eventually be in an unstressed state when the tooth it is urging has reached the position desired. This is thus to a great extent dependent upon the skill of the technician that is producing the appliance. When first installed in the intraoral cavity, the springs are each compressed by contact with the corresponding tooth that is intended to be moved by it, by an amount that is a function of the departure of the position of the tooth from the desired position. With the passage of time, each spring deflects its designated tooth, and at the same time releases the stored potential energy of the spring.
In US 2003/0198915, a method is described for fabricating an orthodontic appliance that includes lingual springs. First, a model of the patient's teeth is cast in the conventional manner. One or more ideal grooves are then cut into the lingual surfaces of the model teeth. The location of the floor of the groove reflects the desired position of these teeth after treatment. The orthodontic appliance is cast on the model using the ideal reference grooves on the tooth to position the tooth contacting portion of each lingual spring.
Other methods make use of a digital model of the dentition rather than a physical model. For example, in U.S. Pat. No. 5,975,893, aligner-based therapy philosophies and digital imaging/computer-driven rapid prototyping methods are combined, in which a set comprising a plurality of aligners is formed for a patient. Each upper and lower set of aligners (where required) is worn for a period of time. Each aligner in the set biases a patient's teeth toward an ideal occlusion more aggressively than the previous aligner, and typically between 15 to 25 progressive aligners may be used in one treatment. Over a period of time, the sequential and progressively biased positioners move teeth from their initial maloccluded positions to a near finished and corrected state. Each aligner appliance generally comprises a U-shaped tray or shell having a trough that fits over the teeth. The tray is formed by sucking a thermo-formable sheet material over the reset stone model of the patient's dentition, using heat, pressure and a vacuum force, simultaneously. A first initial data set corresponding to the patient's current dentition is determined using a scanning technique. A final digital model of the dentition in its desired set up after treatment is designed. Intermediate digital models between the initial and final models are then created, and positive tooth models are fabricated from the digital models using rapid prototyping techniques. A conventional pressure or vacuum molding machine is used to produce the appliances from each of the positive tooth models. In US 2002/0042038, a computer implemented system and method implement a dental treatment plan, by specifying tooth movement pattern using a two-dimensional array.
US 2003/0190575 takes a different approach, and employs orthodontic aligner elements that can be secured to openings in a removable aligner appliance to exert the desired forces on selected teeth. This enables aligners to be used in the treatment of some orthodontic cases. In addition, the aligner elements are removable or adjustable, and enables the forces to be maintained, changed, or reactivated over the course of treatment.